Skeletal muscle insulin resistance is a precipitating factor in the development of obesity, type 2 diabetes, and cardiovascular disease. The cellular mechanisms that are responsible for skeletal muscle insulin resistance remain unknown. The long term objective of this research is to understand the role that myostatin signaling may have in the development of skeletal muscle insulin resistance. The central hypothesis of this project is that myostatin-induced SMAD3 phosphorylation increases IRS-1 serine phosphorylation, resulting in insulin resistance in human skeletal muscle cells. This hypothesis has been formulated on the basis of preliminary data produced in the applicant's laboratory. Using primary skeletal muscle cells from lean and severely obese individuals, the Specific Aims of this project are to: 1) determine if myostatin impairs proximal insulin signaling in human skeletal muscle cells; 2) determine the myostatin post-receptor pathway(s) responsible for insulin resistance; and 3) determine whether myostatin signaling is responsible for impaired insulin action in primary human skeletal muscle cells derived from obese, insulin resistant individuals. It is anticipated these studies will reveal new insights regarding the cellular mechanisms that contribute to insulin resistance in skeletal muscle, and provide the framework for targeted and efficient treatment strategies in the near future.